A photoconductive process for the electrophotographic light-sensitive materials consists of (1) a step of generating electric charges by exposing to light, and (2) a step of transporting the electric charges.
Steps (1) and (2) can be carried out in the same substance with a material such as a selenium light-sensitive plate. However, steps (1) and (2) may be carried out in different substances by using a combination such as amorphous selenium and poly-N-vinylcarbazole. When steps (1) and (2) are carried out in different substances, it is advantageous in that the substances used in the electrophotographic light-sensitive materials can be selected from a wide range of materials. Consequently, the electrophotographic characteristics of the electrophotographic light-sensitive materials, such as sensitivity or acceptable electric potential, etc. are improved. Further, substances suitable for producing the electrophotographic light-sensitive coating layer can be selected from a wide range.
Photoconductive substances in the electrophotographic light-sensitive materials used for the electrophotographic process include selenium, cadmium sulfide and zinc oxide.
In the electrophotographic process, as disclosed in U.S. Pat. No. 2,297,691 by Carlson, the photoconductive material used is comprised of a base coated with a substance which is insulating in the dark, the electric resistance of which varies according to an exposure applied during imagewise exposing to light. This photoconductive material is generally charged first to give uniform surface electric charges in the dark after having carried out dark adaptation for a suitable time. This material is then imagewise exposed to light by a light pattern which has the effect of reducing surface electric charges according to relative energy included in various parts of the light pattern. The residual surface electric charges or latent images formed on the surface of the photoconductive material layer (electrophotographic light-sensitive layer) are brought into contact with a suitable electroscopic indication material, namely, a toner to obtain visible images.
The toner is contained in an insulating solution or a dry carrier. The toner will adhere to the surface of the electrophotographic light-sensitive layer according to the electric charge pattern. The adhered indication material can be fixed by a known means such as heat, pressure or solvent vapor. Further, the latent images can be transferred to a second base (for example, paper or film, etc.). Likewise, the latent images can be transferred to the second base where they are developed. The electrophotographic process is one of such image forming processes.
In such an electrophotographic process, the basic characteristics required for the electrophotographic light-sensitive materials are: (1) the electrophotographic light-sensitive material can be charged in a suitable electric potential in the dark; (2) there is a low level of leakage of electric charges in the dark; and (3) electric charges can be quickly discharged by light irradiation.
It is true that the above described inorganic substances used hitherto have many advantages. However, they also have various disadvantages. For example, selenium widely used at the present sufficiently satisfies the above described requirements (1) to (3). However, it is undesirable because production thereof requires maintaining difficult conditions which increase the production cost. It is difficult to make a long sheet because of the lack of flexibility, and it is necessary to carefully handle the material because it is sensitive to thermal and mechanical impacts. Cadmium sulfide and zinc oxide are used as electrophotographic light-sensitive materials by dispersing them in a resin binder. However, they can not be repeatedly used, because they have mechanical disadvantages such as smoothness, hardness, tensile strength and friction resistance.
In the recent years, in order to remove the disadvantages of these inorganic substances, electrophotographic light-sensitive materials using various organic substances have been proposed and some of them have been put in practical use. For example, there is an electrophotographic light-sensitive material composed of poly-N-vinylcarbazole and 2,4,7-trinitrofluorenon-9-one (U.S. Pat. No. 3,484,237), an electrophotographic light-sensitive material in which poly-N-vinylcarbazole is sensitized with pyrylium salt type dyes (U.S. Pat. No. 3,617,268), an electrophotographic light-sensitive material containing an organic pigment as a main component (U.S. Pat. No. 3,898,084) and an electrophotographic light-sensitive material containing an eutectoid complex composed of a dye and a resin as the main component (U.S. Pat. No. 3,732,180 and 3,684,502), etc.
In these organic electrophotographic light-sensitive materials, though mechanical characteristics and flexibility of the above described inorganic electrophotographic light-sensitive material can be improved to a certain extent, they do not sufficiently satisfy the requirements of electrophotographic light-sensitive materials because they generally have a low light-sensitivity and are not suitable for repeated use.